A continuing program of research is directed toward the development of crashworthy composite-material fuselages for small aircraft. These fuselages are required not only to withstand flight loads and exhibit the required aerodynamic characteristics, but also to protect occupants during crashes more effectively than do conventional fuselages. The design goal for protection against crashes is to limit loads applied to occupants to survivable levels in vertical impacts onto rigid surfaces at a speed of 31 ft/s (9.4 m/s). This vertical impact speed exceeds that specified in current regulatory criteria for small aircraft, but it is a realistic, potentially survivable, impact velocity that has been observed in crashes and in crash tests performed at Langley Research Center.
The design concept that has emerged from this research calls for a structure made from several glass/epoxy-fabric and graphite/epoxy-fabric laminates plus some other materials. The fuselage is divided into four regions, as indicated in the figure. Each region is designed to satisfy a different set of requirements:
- The upper region is made of a stiff sandwich comprising a polyurethane foam core between glass/epoxy-fabric face sheets. This region is designed to enclose and protect the occupants in the event of a crash.
- The outer shell is made from a relatively compliant glass/epoxy-fabric layer that is wrapped around the entire fuselage, enclosing an energy-absorbing structure (the subfloor) beneath a rigid structural floor. The outer shell is designed to have the required aerodynamic shape and to tolerate damage. The outer shell is intended to become deformed upon impact; this deformation, in turn, is intended to initiate crushing of the subfloor.
- The subfloor, described in more detail below, is designed to dissipate kinetic energy through stable crushing, and to maintain adequate post-crash structural integrity.
- The stiff structural floor is designed to react the loads generated by crushing of the subfloor, and to provide a stable platform for attachment of seats and restraints.
Two alternative subfloor designs have been selected: In one design, the main structural components of the subfloor are three laminated glass/epoxy-fabric tubes placed lengthwise along the fuselage. The sizes of the tubes, the number of layers of glass/epoxy fabric, and the orientations of the fibers in the layers must be chosen to optimize the absorption of energy by transverse crushing of the tubes upon impact.
In the other subfloor design, the space below the stiff structural floor is filled with closed-cell polymethylimide foam covered by glass/epoxy-fabric face sheets. The geometry of the foam subfloor was chosen to achieve and to maintain the desired crushing stress level.
The concept has been implemented in 1/5-scale models that have been evaluated in drop tests. The models were dropped from a height of 15 ft (4.6 m) to obtain vertical impact at a speed of 31 ft/s (9.4 m/s) onto a concrete surface. Floor-level accelerations of 125 times normal Earth gravitational acceleration (g) were obtained. (This impact requirement corresponds to a 25-g floor-level acceleration in a full-scale fuselage.) The data from the drop tests indicated that the model with a foam subfloor satisfied the impact design requirement. The results of a finite-element simulation of the impact behavior were found to be well correlated with the corresponding results from the drop tests.
This work was done by Karen E. Jackson and Edwin L. Fasanella of the Vehicle Technology Center of the U.S. Army Research Laboratory for Langley Research Center. L-17835